The number of solar water heaters in use is less than one percent of the total number of water heaters. Of these, the vast majority are solar thermal water heaters. That is, solar energy directly heats water in a collector containing a plurality of pipes containing water. It is also known to convert solar energy to electricity and use the electricity to heat the water. This is solar photovoltaic (PV) water heating.
Solar thermal water heaters are not widely used because of problems of durability, difficulty of installation, weight, and relatively high initial cost. Durability includes problems with freezing, leakage, pump failure, and hard (mineral bearing) water. Installation has often proved difficult, requiring roof penetrations for the plumbing that transports water to and from solar collectors (arrays of pipes containing water that is heated by the sun).
U.S. Pat. No. 4,165,732 (Morin) discloses a solar voltaic preheat tank in combination with an electrically heated main tank. The temperature of the preheat tank is lower than the temperature of the main tank.
U.S. Pat. No. 4,568,821 (Boe) discloses a water heating system having two tanks, a main tank heated by electricity or gas, and a solar thermal preheat tank. The main tank is normally off unless demand exceeds supply from the preheat tank. The main tank is bypassed when the preheat tank is used.
U.S. Pat. No. 4,948,948 (Lesage) discloses an electric water heater having plural heater elements. The elements dissipate different amounts of power from each other and are controlled by a timer for limiting peak demand on a power grid by selecting an element of less than maximum power.
U.S. Pat. No. 5,293,447 (Fanney et al.) discloses a solar voltaic water heater. A microprocessor controls a set of electrical relays that connect the photovoltaic module to several resistive heating elements in a manner that best matches the instantaneous operating characteristics of a photovoltaic module.
In general, a preheating tank is used in the prior art to reduce the load on a main tank by raising the temperature of the water from a cold water supply. While some attempt (Fanney et al.) is made to use photoelectric power efficiently, a point is reached in a single tank system at which available solar energy is not used because reduced demand has permitted the water to reach a maximum safe temperature, e.g. 140° F. (60° C.). Basically, the demand for hot water and the demand for solar energy do not match. A demand for large amounts of hot water, especially around sunrise, can significantly outstrip supply. The only solution is to increase the size of the tank, which increases rather than decreases the amount of energy needed.
In view of the foregoing, it is therefore an object of the invention to provide a solar photovoltaic preheat that absorbs available solar energy.
Another object of the invention is to improve the efficiency of conventionally powered water heaters.
A further object of the invention is to provide a photovoltaic water heater that absorbs energy in proportion to availability.